Background

June 2012: During a packed public meeting, a Nuclear Regulatory Commission inspection team identifies flawed computer modeling by the manufacturer, Mitsubishi Heavy Industries, and resulting excessive vibration among metal tubes inside the plant's steam generators, as the main causes of the problems that led to the shutdown; premature wear is seen in all four of the plant's steam generators, two for each reactor.

October 2012: Edison proposes restarting Unit 2 reactor at reduced power to curb the vibrations.

July: Edison accuses Mitsubishi of "gross negligence" and failing to meet its contractual obligations in a "notice of dispute"; Mitsubishi later calls Edison's contentions "factually incorrect."

Federal regulators will cite the manufacturer and Southern California Edison for a flawed steam-generator design that led to the shutdown of the San Onofre nuclear plant, Edison said in a statement released on Sunday.

The Nuclear Regulatory Commission informed Edison in a letter on Friday that it would issue a notice of non-conformance against the manufacturer of the steam generators, Mitsubishi Heavy Industries, the Edison statement said.

The letter also said NRC would cite Edison for failing to make sure Mitsubishi’s computer modeling and analysis were adequate for the design of the steam generators.

Sunday’s statement from Edison, the nuclear plant’s operator, included comments from Pete Dietrich, an Edison senior vice president and chief nuclear officer.

Dietrich said it is not unusual for the NRC to cite a plant’s licensed operator for problems caused by a vendor or contractor.

“Mitsubishi designed the system,” Dietrich is quoted as saying. “Mitsubishi built the system. Mitsubishi’s system failed. They are the experts. SCE was the customer.”

An NRC spokeswoman said Sunday evening that the agency would make a report available to the public on Monday but declined further comment.

A statement from Mitsubishi said the failure described in the NRC’s notice of non-conformance “had no impact on the root cause” of the “unprecedented” vibration problems that occurred in the nuclear plant’s steam generators.

At issue is a flawed computer code used to model the movement of heat and steam through the generators, detailed last year by NRC inspectors.

They found that the flawed modeling led to flawed design of the nuclear plant’s four steam generators, two for each reactor, installed in a $670 million operation between 2009 and early 2011.

The flawed design, in turn, caused excessive vibration and wear among the thousands of metal tubes inside each of the steam generators, the team said.

A leak in one tube led to a small release of radioactive gas on Jan. 31, 2012, prompting shutdown of the plant’s Unit 3 reactor. Unit 2 already was offline for routine maintenance.

Inspections of both reactor units revealed unexpected tube wear in all four steam generators.

In June this year, Edison announced its decision to shutter the plant permanently, citing costs and regulatory uncertainty.

Edison and NRC are moving to decommission San Onofre, a process that could take many years.

The Mitsubishi statement on Sunday said the steam generator design team, which included “Southern California Edison experts,” “believed at the time that installing additional tube supports had optimized the design to control for vibration.”

The statement from Edison said the NRC’s action did not include financial penalties. It was unclear Sunday whether NRC would take further action against either Mitsubishi or Edison.

The NRC letter, according to Edison’s statement, also said Mitsubishi had used the same flawed computer modeling to design steam generators for four other nuclear plants, although none has shown the vibrations and wear that led to the San Onofre shutdown.

Edison did not identify the four other nuclear plants.

Mitsubishi’s statement said NRC’s “technical expertise and care” during inspections at San Onofre “will have important and significant impacts on (replacement steam generator) design considerations and practices going forward.”

User Agreement

Keep it civil and stay on topic. No profanity, vulgarity, racial
slurs or personal attacks. People who harass others or joke about
tragedies will be blocked. By posting your comment, you agree to
allow Orange County Register Communications, Inc. the right to
republish your name and comment in additional Register publications
without any notification or payment.